Electrical circuits to detect the presence or absence of trains on railroad tracks is well known in the art, such train detecting circuits being used to control the railroad traffic control devices such as signal lights. For full-scale railroads, existing track detection circuits generally comprise a voltage or current source which is electrically connected to both rails at one end of a section (known as a "block") of track. On the opposite end of the block, both rails are electrically connected to some type of receiver or detection device. An electrical power source, typically a battery, connected to the first end of the block provides a current that flows in opposite directions through the parallel rails if no train occupies the block, and so long as the rails have maintained electrical continuity.
When no train occupies this block, the currents applied to the first rail travels from the first end (the power source end) of the block along the first rail to the second end of the block, through a relay coil, and to the second rail. The current then returns on the second rail from the second end of the block to the first end of the block. In this way, the relay coil at the far end of the block is normally energized, thereby making the operation of this system failsafe, so that if a rail should break, there would be no current flow and the relay coil would become de-energized. In typical railroad applications, the supply voltage is in the range of 1-3 volts DC, and the current that flows through the rails when no train is present should be at least 72 milliamperes to energize the relay coil at the far (second) end of the block.
If a train occupies this block, the current supplied by the battery is shunted from the first rail to the second rail by the wheels and axles of the train. When this occurs, there will be essentially no current flowing through the relay coil at the far end of the block, and it becomes de-energized. The contacts of that relay are then used to indicate to the railroad dispatcher that the block is occupied by a train. This information alone does not, however, indicate which direction a train is moving within the block.
To determine the direction of a train as it enters a block, all railroads employ a half-block boundary method which requires additional electrical circuitry and a separate electrical power source and relay for each half of the block. Depending upon which relay is energized for a given half-block, the direction of the train can be determined. The signals from each half-block relay can be transmitted to a third relay which is indicative of whether a train occupies any part of that particular block.
Train detection circuits for model railroads have been available which sense the current supplied by the electrical power supply that provides current to the electric motor of the model railroad engine. The current supplied to turn the motor of the model railroad engine is detected, thereby providing an indication that a model railroad engine is within a particular section or "block" of track. In the case of alternating current model railroads, the train detection circuit simply determines whether or not any current is flowing through that particular block.
In the case of direct current model railroads, a train detecting circuit must be able to work with currents in either direction, since the model railroad engines can operate with either polarity through their motors. As with alternating current model railroads, a simple determination as to whether current exists or not, in either polarity, provides an indication that an electric train engine occupies that particular section of track.
Existing model railroad train detection circuits do not have the capability of determining from which direction a train is entering the block. In addition, existing model railroad train detection circuits provide no indication that a block is occupied when the electric engine is merely resting on that block (and is not moving) because no current is being supplied to its motor.